The present invention relates to a laser printer, digital copier or similar image recorder capable of forming an image on either of opposite sides of a paper sheet, as desired.
Conventional image recorders include one which forms a toner image on at least one side of a paper sheet and fixes the toner image by a fixing device, i.e., a pair of heat rollers while transporting the sheet by a belt and via a guide. This type of image recorder is disclosed in, for example, Japanese Patent Laid-Open Publication No. 63559/1982 and and Japanese Utility Model Laid-Open Publication No. 166650/1983 and will be referred to as a first type of image recorder hereinafter.
Another or second type of conventional image recorder has a rotatable photoconductive element, optical writing means for writing a latent image on the photoconductive element, and developing means for developing the latent image to produce a primary toner image. An intermediate image transfer body is implemented as a belt which faces the photoconductive element and allows a paper sheet to pass through between it and the photoconductive element. Intermediate image transferring means transfers the primary toner image to a paper sheet or the belt as a secondary toner image. Secondary toner image transferring means is located downstream of the intermediate image transferring means for transferring the secondary toner image to a paper sheet. A fixing device has heat rollers for fixing the toner image by passing a paper sheet driven out from the belt therethrough. The second type of image recorder is capable of recording an image on one or both sides of a paper sheet. In a front record mode, the primary toner image is transferred from the photoconductive element to the front or upper surface of of a paper sheet on the belt by the intermediate image transferring mans. In a rear record mode, the primary toner image is once transferred to the belt as a secondary toner image by the intermediate image transferring means, and then the secondary toner image is transferred to the rear or lower surface of the paper sheet on the belt by the secondary toner image transferring means. Further, in a two-side record mode, the secondary toner image is transferred to the belt in the same manner as in the rear record mode, then a primary toner image is formed on the photoconductive element in the same manner as in the front record mode, and then the primary and second toner images are respectively transferred to the front and the rear of a paper sheet being transported by the belt.
Both the first and second types of image recorders are operable not only with a toner of one color but also with a plurality of toners of different colors. By using toners of different colors, each of the two types of image recorders implements multicolor or fullcolor recording in which superposed toner images of different colors are formed on the photoconductive element or the belt. Hence, three different record modes are available with such an image recorder, i.e., a front record mode for forming a monocolor toner image or a multicolor or fullcolor toner image on the front of a paper sheet, a rear record mode for forming the toner image on the rear of a paper sheet, and a two-side record mode which is the combination of the front and rear record modes. By selecting any one of such record modes, it is possible to form an image on one or both sides of a paper sheet. This kind of system does not have to turn over a paper sheet and is, therefore, practicable with a simple and contact arrangement.
For the transfer of a toner image, an electrostatic field is generated by corona transfer, belt transfer, or roller transfer. Among them, the roller transfer or so-called bias roll transfer uses a conductive rubber roller or a bias roller made up of a conductive rubber roller and a dielectric film covering the rubber roller. The bias roller is applied with a voltage and pressed against the side of a paper sheet opposite to the image carrying side to generate a field. This kind of image transfer has an excellent image transfer efficiency, as taught in, for example, Japanese Patent Laid-Open Publication Nos. 209471/1989, 28879/1989, and 28880/1989.
In each of the first and second types of image recorders, a paper sheet carrying a toner image on one side thereof is passed through a pair of rotatable fixing rollers which face each other. When such fixing rollers fail to nip the leading edge of the paper sheet evenly in the widthwise direction of the latter, the leading edge of the paper sheet is apt to fold or crease or, in the worst case, jam the transport path. To eliminate such an occurrence, an upper guide and a lower guide are usually located just in front of the fixing rollers so as to guide the paper sheet smoothly to the rollers. However, the problem is that the leading edge of a paper sheet, except for paper sheets which are thick and have flat and elastic leading edges, is likely to become unstable, depending on the transport condition. It is likely, therefore, that the guides rub against a non-fixed toner image carried on a paper sheet, critically degrading the toner image. When the secondary toner image transferring means included in the second type of image recorder is implemented as a bias roller, it is necessary that in the front record mode the bias roller be held in contact with the belt. Should the primary toner image be transferred to a paper sheet or to the belt (rear record mode) while the bias roller is in contact with the belt, the bias roller would rub against the primary tonr image.